It has been proposed that the development of epileptiform activity, the transition between interictal spikes and seizures, and the spread of seizures from abnormal to normal brain are all, at least partially, due to differences in the way excitatory and inhibitory synaptic function respond to moderate and high frequency activation. This grant proposes to first test the hypothesis that under identical physiological conditions, cortical inhibitory synapses decrease in efficacy while cortical excitatory synapses potentiate. If found to be true, new hypotheses will then be developed to explain the mechanisms which underlie such changes in synaptic efficacy and these will be tested using dissociated cell cultures of mammalian neocortex and hippocampus as model systems. Intracellular recording from both the pre and postsynaptic neurons of pairs which are coupled by excitatory and inhibitory connections will be performed with both conventional microelectrodes and WC patch microelectrodes. The presynaptic and/or postsynaptic factors involved in inhibitory synaptic decrement will be determined. Postsynaptic mechanisms such as changes in receptor sensitivity during repetitive activation (desensitization), a shift in the C1 equilibrium potential causing an apparent change in IPSP efficacy, or activation of other channels by the inhibitory neurotransmitter at higher concentrations or during prolonged exposures to the receptor will be investigated. Presynaptic mechanisms such as the ability of inhibitory interneurons to fire with high frequency repetitive activation, the characteristics of neurotransmitter release at excitatory and inhibitory synapses, and the ability of GABA to activate presynaptic GABAA or GABAB receptors which then act as presynaptic inhibitors will also be examined. Excitatory and inhibitory synapses will be analyzed with the techniques of the quantal analysis and the release characteristics of excitatory and inhibitory neurotransmitters will be compared to determine whether release is adequately described by Poisson or binomial statistics, and how changes in divalent cation concentrations influence release of neurotransmitter. The quantal analysis will also be used to further define the pre or postsynaptic mechanisms which underlie frequency dependent changes in inhibitory synaptic efficacy. It is hoped that an increased understanding of the physiological regulation of excitatory and inhibitory synaptic function in mammalian cortex will contribute toward the development of improved strategies for treating epilepsy, and especially for preventing the transition between relatively benign interictal abnormalities and the much more disruptive seizure.